EP1428017B1 - Micromechanical heat conductivity sensor having a porous cover - Google Patents

Description

The present invention relates to a micromechanical thermal conductivity sensor. In particular, it relates to a micromechanical thermal conductivity sensor which comprises a thermally insulated membrane formed by recessing a base plate having poor thermal conductivity, at least one heating element applied to the membrane, at least one temperature dependent electrical resistance applied to the membrane for measuring the temperature of the membrane (T _M ), and at least another temperature-dependent electrical resistance applied to the base plate outside the membrane for measuring the ambient temperature (T _U ). Furthermore, the invention relates to methods for producing the thermal conductivity sensor according to the invention, as well as its use.

The measurement of the thermal conductivity is often for analysis of gases, in particular the quantitative analysis of two-component Gas mixtures used.

In principle, the thermal conductivity of a gas or gas mixture varies with the mass of the gas molecules, their concentration and the temperature. If the (mixed) thermal conductivity of the gas or gas mixture is measured at a known gas temperature and known components, the concentrations of individual components of the gas mixture can be determined precisely via the thermal conductivity. Above all, the concentrations of hydrogen (H ₂ ) and helium (He) may be mixed with other gases such as air, oxygen (O ₂ ), nitrogen (N ₂ ), ammonia (NH ₃ ), argon (Ar), carbon dioxide (CO ₂ ), carbon monoxide (CO), chlorine (Cl ₂ ), hydrogen sulfide (H ₂ S), methane (CH ₃ ), nitrogen monoxide (NO), nitrous oxide (N ₂ O) and water vapor (H ₂ O) are measured well because these gases have a particularly high thermal conductivity compared to the other gases. Thus, the thermal conductivity of hydrogen (H ₂ ) λ _hydrogen = 0.84 Wm ^-1 K ^-1 and helium (He)) λ _helium = 0. 6 Wm ^-1 K ^-1 , while the thermal conductivity of air λ _air = 0 , 012 Wm ^-1 K ^-1 is smaller by about a factor of 5-7.

To measure the thermal conductivity of a body is brought to a temperature T _K , which is higher than the ambient temperature T _U. A gas (mixture) surrounding the body is generally at ambient temperature. If, for example, the temperature difference ΔT = T _K -T _{U is} kept constant, then the heating power P _H required for this purpose is a measure of the thermal conductivity of the surrounding gas or gas mixture. The heating power P _H is directly proportional to the temperature difference .DELTA.T held constant, the proportionality constant resulting from the product of the thermal conductivity λ and a (constant) geometry factor K dependent on the measuring device. This relationship is described in equation (1): P H = K λ ΔT where P _{H is} the heating power, ΔT the temperature difference, K the geometry factor and λ the thermal conductivity.

To determine the thermal conductivity are lately increasingly micromechanical thermal conductivity sensors Silicon base developed. In contrast to conventional thermal conductivity sensors These sensors stand out a low power consumption, which is essentially not is greater than the power consumption of the signal processing necessary electronics. In addition, the miniaturized Sensors with short response times (time constants), which in the conventional sensors in general only by accepting a forced influx with the sample gas, d. H. to achieve a flow dependence are. However, this ultimately makes a universal use impossible. Finally, such micromechanical Silicon-based thermal conductivity sensors economical in the production, because of common manufacturing processes recourse to integrated semiconductor components can.

The construction of a typical conventional micromechanical Silicon-based thermal conductivity sensor is shown in FIG. 1 and will be discussed below.

A fundamental problem in the measurement of thermal conductivity of a surrounding gas is by convection of the Gases caused heat transfer with a concomitant Falsification of the actual thermal conductivity value. One convective heat transport can result from external gas movements, which are noticeable in the sensor, but also from the for measuring necessary temperature difference between heating element or membrane and gas.

Although a forced flow of the thermal conductivity sensor be quite desirable with the gas to be measured a fast gas exchange and thus short response times however, micromechanical thermal conductivity sensors are used with very low measuring volumes and one thus allowing inherent fast gas exchange of any convective heat transport considered disadvantageous.

To prevent the convective heat transfer, the Sensor usually used so that the gas volume in the membrane remains at rest. This can be z. B. by Covering the membrane can be achieved with a cover plate.

For example, DE 37 115 11 C1 describes a micromechanical Thermal conductivity sensor for measuring the thermal conductivity a gas mixture in which on a support plate made of silicon, an insulator layer is applied on the by vapor deposition or sputtering meandering thin film resistors are applied. In the field of thin-film resistors the insulator layer is undercut, leaving a pit in the Support plate is formed, which is the lower part of the measuring chamber of the sensor. On the carrier plate with the thin-film resistors rests a cover plate of silicon, in the height the thin - film resistors a pit is etched, the forms the upper part of the measuring chamber. The cover plate has an opening which, as a diffusion channel, enters the gas mixture allows for the measuring chamber. The gas exchange in the bottom Pit of the measuring chamber takes place through breakthroughs in the Insulator layer.

A disadvantage of this thermal conductivity sensor is that the In any case, so choose dimensions of the diffusion channel are that on the one hand by a large opening the gas in the Measuring chamber replaced as quickly as possible by diffusion but on the other hand gas movements outside the measuring chamber occur, not be transferred to the measuring chamber, which requires a small opening. However, this task can only solved as a compromise between the two requirements be a given interpretation of the diffusion channel in general universal use no longer possible.

Further thermal conductivity sensors with a cover are known from JP1193638, DE 3837951 and US 5535614 known.

Object of the present invention is to overcome the disadvantages of known in the art micromechanical thermal conductivity sensors to overcome. In particular, should be a universal be specified usable thermal conductivity sensor, in which a convective heat transport through the to be measured Gas or gas mixture is largely avoided.

This object is achieved by the characterizing features of the claim 1 fulfilled. Advantageous embodiments of the invention are indicated by the features of the dependent claims.

According to the invention, a micromechanical thermal conductivity sensor which one by recess of a thermally conductive base plate formed thermally isolated membrane, at least one applied on the membrane Heating element, at least one applied to the membrane temperature-dependent electrical resistance for measurement the temperature of the membrane, as well as at least one other outside the membrane on the base plate applied temperature-dependent electrical resistance for measuring the ambient temperature includes, the membrane being supported on one or both sides thereof the gas exchange by diffusion permitting porous cover plate is covered, between membrane and porous Cover plate a cavity is recessed, the porous cover plate is fixed to the base plate and the porous Cover plate made of a porous ceramic material or at least partially made of porous silicon. The inventive Thermal conductivity sensor can thus with one or two porous Cover plates be equipped, d. H. one or two cavities between membrane and cover plate (s) have.

Advantageously, by the porous (n) cover plate (s) a diffusion of the surrounding gas or gas mixture in the allows the measuring space forming cavity without doing that Measurement result falsified by a convection flow of the gas is. Due to the small size of the cavity (s), in particular by their low height, which is due to the Temperature difference between membrane or heating element and Cover plate caused convective heat transport in the direction of a Cover plate significantly reduced, leaving a convection flow of the gas to be measured in a cavity in total minimized and substantially prevented. The response time The sensor is replaced by the fast gas exchange greatly reduced. Furthermore, the sensitivity of the Sensors are increased because of the minimized convection flow the gas or gas mixture to be measured the possibility exists, a relatively large temperature difference between Insert membrane and sample gas. In addition there is the possibility of the sensor regardless of the installation situation to be used universally.

As shown above, the porous cover plate can be placed on a or both sides of the (flat) membrane are applied. If the porous cover plate on the one side of the base plate applied, which the recess for forming the Membrane, this recess forms after application of the Cover plate a cavity, which uses as a measuring space for the sensor becomes. Will the porous cover plate on this side applied opposite side of the base plate, so must the porous cover plate one of the membrane arranged opposite Have recess which after applying the Cover plate forms a cavity. This cavity then forms a Measuring chamber for the thermal conductivity sensor. The sensor Thus, either via one or two cavities (or Measuring chambers) for the gas (mixture) to be measured.

The base plate is preferably made of silicon. This has the advantage that to further processing on in the Semiconductor technology used methods, such as Vapor deposition, sputtering, photolithographic Method, etching method and passivation method recourse can be. Such is a cost-effective production the sensors possible. In particular, from a single Silicon wafer a variety of sensors according to the invention getting produced.

The porous cover plate is preferably made of SiC and Al ₂ O ₃ .

It is preferable that the thermal expansion coefficient of the ceramic material the thermal expansion coefficient of silicon is substantially equal, or this at least adjacent. In this case, cover plate (s) and base plate or membrane a same thermal Expansion, which advantageously thermally induced Tensions after application of the cover plate on the Base plate can be avoided or minimized.

The thickness of the base plate used for the production of the sensor is preferably in the range of 200-600 μm, while the membrane preferably has a thickness in the range of 0.6-2 μm. Further, the membrane is characterized by a surface which is preferably in the range of 0.25 - 4 mm ² .

If the pores of the porous material of the cover plate are sufficient can be chosen small, the passing Gas (mixture) in a very advantageous manner during diffusion be filtered. Depending on the pore size of the porous material can by this filtering foreign substances, for example Soot and dust particles or microbiological contaminants, be removed from the gas (mixture).

The heating element and the temperature-dependent electrical resistances are preferably silver (Ag), gold (Au), nickel (Ni) or platinum (Pt) manufactured. In a particularly advantageous Embodiment of the invention is the cross section of the heating element in the contact area and in the Heating zone varies in size. This can be advantageous in the Sintering the cover plate can be used on the base plate.

The temperature-dependent electrical resistances or the heating element can be protected by a passivation layer against the influence of chemically aggressive gases or gas mixtures. As substances for the passivation layer, silicon compounds such as silicon oxide (SiO ₂ ) and silicon nitride (Si ₃ N ₄ ) are particularly suitable.

The thermal conductivity sensor according to the invention can be produced by placing the porous cover plate on the base plate is glued. An adhesion of base and cover plate is particularly suitable if the materials used a different thermal expansion coefficient because an adhesive bond is capable of compensate for a different thermal expansion. This avoids thermally induced stresses in the connected Materials. As adhesives, in particular silver-filled Adhesives with adjusted thermal expansion coefficients be used.

With only a small difference in the thermal expansion coefficient between the example of a porous ceramic material existing porous cover plate and the base plate is also a direct cladding of the base plate with ceramic by sintering possible. For this purpose will be the application of the material to be sintered the porous cover plate a cover layer to be removed after sintering Formation of the cavity in the porous cover plate on the base plate applied.

The heat required for the sintering process can be supplied from the outside become. However, there is also the possibility that the Sintering process solely by the heating heat of the heating element is effected. In this case, the diameter of the heating element, For example, a platinum wire, in the heating zone essential thicker than in the area of its contact. In Particularly advantageously, the cover layer removed by the heating heat of the heating element and so a cavity defined dimensions are formed.

Furthermore, the thermal conductivity sensor according to the invention while avoiding the joining process between the base and cover plate be made monolithic. For this is a Layer of porous silicon on the actual sensor base material made of silicon, this being done, that between the subsequently formed membrane and the Layer of porous silicon, a cavity is formed.

Fig. 1 zeigt den Aufbau eines herkömmlichen mikromechanischen Wärmeleitfähigkeitssensors jeweils in Draufsicht (unterer Teil) und im Querschnitt längs der angegebenen Schnittlinie (oberer Teil);

Fig. 2 zeigt eine Ausführungsform des erfindungsgemäßen Wärmeleitfähigkeitssensors, bei dem beiderseits der Membran eine poröse Deckplatte auf die Grundplatte aus Silizium aufgeklebt ist;

Fig. 3 zeigt eine weitere Ausführungsform des erfindungsgemäßen Wärmeleitfähigkeitssensors, bei dem beiderseits der Membran eine poröse Deckplatte auf die Grundplatte aus Silizium aufgesintert ist; und

Fig. 4 zeigt zwei weitere Ausführungsform des erfindungsgemäßen Wärmeleitfähigkeitssensors, bei dem beiderseits der Membran eine poröse Deckplatte aus Silizium auf die Grundplatte aus Silizium aufgesintert ist.

In the following, with reference to the accompanying drawings, several embodiments of the thermal conductivity sensor according to the invention, and methods for their preparation are shown.

Fig. 1 shows the structure of a conventional micromechanical thermal conductivity sensor respectively in plan view (lower part) and in cross section along the indicated section line (upper part);

Fig. 2 shows an embodiment of the thermal conductivity sensor according to the invention, in which both sides of the membrane, a porous cover plate is adhered to the base plate made of silicon;

Fig. 3 shows a further embodiment of the thermal conductivity sensor according to the invention, in which a porous cover plate is sintered on both sides of the membrane on the base plate made of silicon; and

Fig. 4 shows two further embodiment of the thermal conductivity sensor according to the invention, in which a porous cover plate made of silicon is sintered on both sides of the membrane on the base plate made of silicon.

As can be seen from the lower part of Fig. 1, in a typical conventional micromechanical thermal conductivity sensor, a heating element 3 in the form of a Pt-heating wire and a temperature-dependent electrical resistance 5 for detecting the temperature of the membrane T _M meandering on the base plate 2 and Membrane 1 is arranged from silicon. Outside the membrane 1 is another temperature-dependent electrical resistance 16 for detecting the ambient temperature T _U. The base plate 2 forms, together with the applied electrical structures, a silicon sensor chip for measuring the thermal conductivity. In the upper part of Fig. 1 it can be seen that the membrane 1 is formed as a recess of the base plate 2.

To measure the thermal conductivity of the surrounding gas or gas mixture, for example, the heat output of the heating element required to keep the difference between T _M and T _U constant is determined. Due to the low mass of the thin membrane and the materials deposited thereon, very low thermal time constants, typically on the order of milliseconds and less, can be achieved. Moreover, because of the thin membrane, the heat dissipation through the surrounding gas is substantially greater than over the membrane material itself, resulting in a high sensitivity of the sensor to changes in the thermal conductivity of the surrounding gas. The high sensitivity of the sensor also opens up the possibility of reducing overall heat dissipation, ie to lower the temperature difference ΔT between the membrane and the environment, which is reflected in a reduction in the electrical power loss of the sensor.

In Fig. 2 is an embodiment of the thermal conductivity sensor according to the invention shown. On both sides of the Silicon sensor chips, resulting from the base plate 2 with the applied current-conducting structures 3, 5 composed, is applied a cover plate 8 made of porous ceramic. The Recess of the base plate 2 forms together with the cover plate 8 a cavity 6, which is used as a measuring space for the sensor becomes. On the other side of the membrane 1 is a Recess formed in the cover plate 8, which after application the cover plate 8 on the base plate 2 a cavity. 4 forms, which serves as another measuring space for the sensor. The Both cover plates 8 are each on the silicon sensor chip glued. The adhesive layers 9 are suitable, any differences in the thermal expansion of the ceramic cover plates 8 and compensate for the silicon sensor chip.

Fig. 3 shows a further embodiment of the invention Thermal conductivity sensor. Here is the both sides of Membrane 1 a cover plate made of porous ceramic material sintered on the silicon sensor chip. The sintering of the Cover plates 8 is different because of the lack of possibility thermal expansions between cover plates and Silicon sensor chip to compensate, only at very similar or same thermal expansion coefficient of the ceramic Material of the cover plates and the silicon sensor chip attached. Before applying a paste or dispersion of the ceramic material for the cover plates 8 on the silicon sensor chip is in the range of the heating element 3 and / or the Membrane 1 applied a cover 22, which after the Sintering of the ceramic material is removed. In the place of previous cover layer 22 remain the cavities 4, 6 left.

This manufacturing sequence is illustrated in FIG. 3. Fig. 3a) shows the silicon sensor chip 2. Fig. 3b) shows the silicon sensor chip 2 with applied cover layer 22. Fig. 3c) shows the silicon sensor chip 2 with applied cover layer 22, as well as on both sides of the silicon sensor chip applied ceramic material of the cover plates 8. Fig. 4d), finally, shows after the sintering and removal the cover layer 22 resulting cavities 4, 6th

Both the sintering of the ceramic material of the cover plates 8, as well as the removal of the cover layer 22 may by the Heating heat of the Pt heating wire 3 are effected. There is the cover layer of a less heat-resistant, for example organic (sacrificial) material. About the layer thickness this decomposing material during the sintering process can be a defined cavity between Pt heating wire and porous ceramic enclosure are created. The Pt heating wire 3 has a different cross-section for this purpose Contacting and in the area of the heating zone. To the electric Contacting is a bonding connection 11 on the silicon sensor chip 2 applied.

Fig. 4 shows two further embodiments of the invention Thermal conductivity sensor. In these embodiments is replaced by a porous ceramic for the cover plates. 8 used porous silicon. Because of the same thermal Expansion coefficients of cover plates and silicon sensor chip are thermally induced material stresses from the outset avoided.

Fig. 4a) shows the silicon sensor chip 2. In Fig. 4b) is a Silicon wafer 12 shown with a porous region 13, wherein the silicon wafer 12 in the porous region 13 a recess having. The silicon wafer 12 is on both sides of Membrane 1 is added to the silicon sensor chip 2, wherein the porous Region 13 in each case over the membrane 1 comes to rest.

4 c) shows a variant in which the silicon wafer 12 on the side of the membrane 1, on which the recess of the Silicon sensor chips 2 is located so arranged that the Cavities of the silicon wafer 12 and the silicon sensor chip. 2 form a common cavity with larger volumes.

Fig. 4d) shows a further variant in which the silicon wafer 12 on the side of the diaphragm 1, on which the recess of the silicon sensor chip 2 is arranged so that the cavity of the silicon sensor chip 2 unchanged remains. The arrangement of the silicon wafer 12 on the respectively opposite membrane side is in both variants of the Fig. 4c) and 4d) the same.

In a particularly advantageous modification of the above two mentioned variants, the complex joining process can be avoided be by already on the sensor chip 2 a layer of porous silicon is generated, wherein between the membrane. 1 and the porous silicon layer is formed a cavity. This creates a monolithic thermal conductivity sensor.

The micromechanical Wärmeleitfähigkeitssen-Sors invention is used advantageously for the analysis of gases and gas mixtures, which are analyzed in particular with regard to their nature and concentration. In particular, binary gas mixtures are suitable for a quantitative analysis. Because of their comparatively high thermal conductivity, hydrogen gas (H ₂ ) and helium (He) can be analyzed easily and quickly.

LIST OF REFERENCE NUMBERS

1

membrane

2

baseplate

3

heating element

4

cavity

5

temperature-dependent electrical resistance to Detection of the membrane temperature

6

cavity

7

temperature-dependent electrical resistance to Detection of the ambient temperature

8th

porous cover plate

9

adhesive layer

10

covering

11

Bond connection

12

Silicon wafer with porous area

13

porous area of the silicon wafer

Claims (20)

1. Micromechanical thermal conductivity sensor, which has a thermally insulated membrane (1) which is formed by a cut out in a poorly thermally conductive baseplate (2), at least one heating element (3) which is fitted on the membrane (1), at least one temperature-dependent electrical resistor (5) which is fitted on the membrane in order to measure the temperature of the membrane (TK), as well as at least one further temperature-dependent electrical resistor (7) which is fitted on the baseplate away from the membrane in order to measure the ambient temperature (TU), with the membrane (1) being covered on one or both of its faces by a porous covering plate (8) which allows gas exchange by diffusion, with a cavity (4, 6) being cut out between the membrane (1) and the porous covering plate (8), the porous covering plate (8) being mounted on the baseplate, and the porous covering plate (8) being manufactured from a porous ceramic material, or at least partially from porous silicon.
2. Micromechanical thermal conductivity sensor according to Claim 1, characterized in that the porous covering plate (8) has a cutout (4) which forms the cavity.
3. Micromechanical thermal conductivity sensor according to Claim 1 or 2, characterized in that the baseplate (2) is manufactured from silicon.
4. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the porous covering plate (8) is manufactured from SiC or A1203.
5. Micromechanical thermal conductivity sensor according to Claim 4, characterized in that the thermal coefficient of expansion of the ceramic material is essentially the same as or similar to the thermal coefficient of expansion of silicon.
6. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the membrane (1) has a surface in the region of 0.25 to 2 µm.
7. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the membrane (1) has a thickness in the range from 0.6 to 2 µm.
8. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the cutout in the porous covering plate (8) has a depth in the range from 0.1 to 2 mm.
9. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the porous covering plate (8) provides filtering for the surrounding gas or gas mixture.
10. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the heating element (3) and the temperature-dependent electrical resistors (5, 7) are manufactured from silver (Ag), gold (Au), nickel (Ni) or platinum (Pt).
11. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the heating element (3) has a different cross section for making contact and in the area of the heating zone.
12. Micromechanical thermal conductivity sensor according to one of the preceding claims, characterized in that the temperature-dependent electrical resistors (5, 7) are protected against the influence of chemically aggressive gases or gas mixtures by means of a passivation layer, in particular a silicon compound such as silicon oxide (SiO₂) and silicon nitride (Si₃N₄).
13. Method for production of the micromechanical thermal conductivity sensor according to one of Claims 1-12, characterized in that the porous covering plate (8) is adhesively bonded to the baseplate (2).
14. Method for production of the micromechanical thermal conductivity sensor according to one of Claims 1-12, characterized in that the porous covering plate (8) is sintered to the baseplate (2).
15. Method according to Claim 14, characterized in that a covering layer (10), which can be removed after sintering, is applied to the baseplate (2) before the application of the material of the porous covering plate (8) to be sintered.
16. Method according to Claim 14 or 15, characterized in that the heat which is required for sintering is produced by the heating element (3).
17. Method according to Claims 15 and 16, characterized in that the covering layer (10) is removed by the heat produced by the heating element (3).
18. Method for production of the micromechanical thermal conductivity sensor according to one of Claims 1-12, characterized in that the thermal conductivity sensor is manufactured monolithically by producing a layer of porous silicon, a membrane adjacent to the layer of porous silicon, as well as a cavity which is arranged between the layer of porous silicon and the membrane, on the baseplate.
19. Use of the micromechanical thermal conductivity sensor according to one of Claims 1-12 for analysis, in particular quantitative analysis, of gases and gas mixtures.
20. Use of the micromechanical thermal conductivity sensor according to Claim 19 for analysis of a gas mixture which contains hydrogen gas (H₂) and/or helium (He).

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